In 3GPP (3rd Generation Partnership Project) which is a project aiming to standardize a mobile communication system, the introduction of Device to Device (D2D) communication is discussed as a new function after Release 12 (see Non Patent Literature 1).
In the D2D communication, a plurality of adjacent user terminals perform direct device-to-device communication without passing through a network. On the other hand, in cellular communication which is normal communication in a mobile communication system, a user terminal performs communication through a network.
It is noted that cases are assumed where assignment of a radio resource used for the D2D communication is led by a network apparatus such as a base station and the assignment thereof is led by a user terminal that performs the
D2D communication. As a result of the user terminal itself performing the D2D communication assigning the radio resource, it is possible to reduce a load of the base station.
In the case where the assignment of the radio resource used for the D2D communication is led by the user terminal, a case is assumed where there are a plurality of D2D groups to which a user terminal that assigns the radio resource (hereinafter, “scheduling terminal”) and a user terminal to which the radio resource is assigned (hereinafter, “non-scheduling terminal”) belong. In this case, the non-scheduling terminal that belongs to the plurality of D2D groups and performs the D2D communication is assigned the radio resource from each scheduling terminal belonging to each D2D group. Generally, the scheduling terminals respectively independently assign the radio resource, and thus, the radio resources assigned to the non-scheduling terminals may overlap to cause trouble in the D2D communication.
Further, even when the non-scheduling terminal is not assigned the radio resource from the plurality of scheduling terminals, if a plurality of D2D groups mixedly exist, then the overlapping of the radio resources assigned by the plurality of scheduling terminals may still cause trouble in the D2D communication. In particular, the larger the number of D2D groups, that is, the larger the number of scheduling terminals, the higher the possibility that the radio resources overlap.